Caption: Solar sail spaceship. Computer artwork of a spacecraft using a solar sail as a means of propulsion. These theoretical vessels would have thin, sail-like membranes, many kilometres wide, which would be used to sail using sunlight. The accumulative pressure of the countless photons of light, emitted by stars such as our sun, would gradually impart momentum to the spacecraft. The acceleration would be very low but continuous. Very high speeds could eventually be attained. An alien planet and its moons are also seen.